Servo system with reduction of transient error and settling time

ABSTRACT

A servo system wherein an actuating signal obtained by the subtraction of a controlled variable delivered from a controlled system from a secondary command variable is applied to the controlled system, the actuating signal is added to the command to provide a secondary command variable, and the command is compared with the controlled variable in such a manner that when they are coincident with each other, the control is effected so as to change the secondary command variable to a predetermined value, thereby reducing both the transient error and the transient or settling time to a minimum and consequently decreasing the steady state error to a minimum.

BACKGROUND OF THE INVENTION

The present invention relates to a servo system with a positive feedbackloop.

There has been devised and demonstrated a servo system which, inaddition to a negative feedback loop for applying to a controlled systeman actuating signal which is the difference between a controlledvariable, which is an output from the controlled system, and a secondarycommand variable, so as to approach the controlled variable to thesecondary command variable, includes a positive feedback loop forgenerating the secondary command variable by adding to the command thedifference obtained by subtracting the controlled variable from thesecondary command variable. When this servo system is applied to aone-lag controlled system, the steady state error may be reduced tozero, but in response to a step change in the command, the response ofthe controlled variable becomes oscillatory. When a one-lag system witha large time constant is inserted into the positive feedback loop inorder to avoid this oscillation, the settling time becomes longer.

As described above, the prior art servo system has the defects that thetransient error is large and that the settling time is longer eventhough the steady state error may be reduced zero. Furthermore, whendisturbance is applied to the controlled system, the transient error islarge and the settling time is longer even though the steady state errorbecomes zero.

SUMMARY OF THE INVENTION

Accordingly, one of the objects of the present invention is to provide aservo system which may reduce both the transient error and the settlingtime to a minimum and may also decrease the steady state error to aminimum.

Another object of the present invention is to provide a servo systemwherein when a controlled variable approaches a command, a secondarycommand variable which is obtained by the addition of an actuatingsignal to the command, is changed so as to compensate for the overshootof the secondary command variable, thereby reducing both the transienterror and the settling time to a minimum.

Briefly stated, the servo system in accord with the present inventioncomprises a command generating means, a secondary command variablegenerating means for generating a secondary command variable from acommand and an actuating signal, an actuating signal generating meansfor generating said actuating signal by the subtraction of a controlledvariable from the secondary command variable, a comparator means forcomparing the command with the controlled variable, and means responsiveto the output from said comparator means for controlling the secondarycommand variable generating means, thereby changing the secondarycommand variable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art servo system;

FIGS. 2 and 3 show waveforms used for the explanation of the servosystem shown in FIG. 1;

FIG. 4 is a block diagram used for the explanation of the underlyingprinciple of the present invention;

FIG. 5 shows wave forms used for the explanation of the mode ofoperation of the servo system shown in FIG. 4;

FIG. 6 is a block diagram of a first embodiment of a servo system inaccord with the present invention;

FIGS. 7 and 7A-7C are detailed circuit diagrams thereof;

FIG. 8 is wave forms used for the explanation of the mode of operationof the servo system shown in FIG. 7;

FIG. 9 is a block diagram of a second embodiment of a servo system inaccord with the present invention;

FIGS. 10 and 11 show wave forms used for the explanation of the mode ofoperation of the servo system shown in FIG. 9;

FIGS. 12 and 12A-12E are detailed circuit diagrams of the servo systemshown in FIG. 9;

FIG. 13 is a block diagram of a third embodiment of a servo system ofthe present invention, which is a modification of the servo system shownin FIG. 9; and

FIGS. 14 and 14A-14C are detailed circuit diagrams thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Prior to the description of the preferred embodiments of the presentinvention, a prior art servo system will be described with reference toFIGS. 1-3. In FIG. 1, the command a from means 1 for generating acommand (to be referred to as "the command generating means" hereinafterin this specification) is transmitted through an adder 2 and asubtractor 3 to a controlled system 4. The output doof the controlledsystem 4 is fed back to the subtracter 3 so as to be subtracted from theoutput b of the adder 2. The output c of the subtracter 3 is applied tothe controlled system 4 and fed back to the adder 2 so as to be added tothe command a.

When the controlled system 4 in the above servo system is a one-lagsystem and when the output a of the command generating means 1 variesstepwise as shown in FIG. 2, the output d of the controlled system 4oscillates. When the steady state is reached, the difference between theoutput d and the output a of the command generating means 1 becomeszero. In this case, the output b of the adder 2 is so slightly higherthan the command a that the difference between the command a and theoutput d of the controlled system becomes zero. The time constant of theoscillatory damping of the output d of the controlled system 4 isgreater than that of the controlled system 4. When the time constant ofthe one-lag system in the adder 2 is increased so as to increase theretardation or damping effect in order to avoid such oscillation, thetime required for reaching the steady-state value becomes longer. Asdescribed above, in the servo system having not only the negativefeedback loop but also the positive feedback loop, the stationarydeviation between the output d of the controlled system 4 and thecommand a becomes zero, but there are defects that the transient erroror deviation is greater and that the response time is slow. Furthermoreas shown in FIG. 3, in response to the disturbance e to the controlledsystem 4, the output d varies. The stationary error or deviation becomeszero, but the transient error or deviation becomes greater and the timerequired to reach the steady-state value becomes longer.

Next referring to FIG. 4, the underlying principle of the presentinvention will be described. The command A or the output of a commandgenerating means 5 is applied to a means 6 for generating a secondarycommand variable (to be referred to as "the secondary command variablegenerating means" hereinafter in this specification). The actuatingsignal C or the output of an actuating signal generating means 7 isapplied not only to the secondary command variable generating means 6but also to a controlled system 8. The controlled variable D or theoutput of the controlled system 8 is fed back not only to the actuatingsignal generating means 7 but also to a comparator 9. The actuatingsignal generating means 7 subtracts the controlled variable D from thesecondary command variable B and applies the difference between them tothe controlled system 8. The comparator 9 compares the command A fromthe command generating means 5 with the controlled variable D andapplies the output F to the secondary command variable generating means6 so as to manupulate the secondary command variable B.

Referring further to FIG. 5, when the command A changes stepwise, thesecondary command variable B increases rapidly, but the increase in thecontrolled variable D or the output from the controlled system 8, whichis a one-lag system, lags behind that of the secondary command variableB. When the comparator 9 detects that the command A is not coincidentwith the controlled variable D, it gives no output, but when they arecoincident with each other, it generates the signal F. Therefore thesecondary command variable generating means 6 interrupts the actuatingsignal C or the output from the actuating signal generating means 7 soas to cause the secondary command variable B to reach the command A.Alternatively, it converts the actuating signal C into a constant, whichin turn is added to the command A, whereby the difference between thecommand A and the controlled variable D may be reduced to zero. When thedisturbance E is impressed to the controlled system 8, the differencebetween the command A and the controlled variable D may be reduced tozero in a manner substantially similar to that described above.

The underlying principle of the present invention described above may beembodied into a practical system or a first preferred embodiment of thepresent invention with the use of analog circuits as shown in FIG. 6.The command generating means 5, the actuating signal generating means 7,the controlled system 8 and the comparator 9 have the functionsdescribed above. The secondary command variable generating means 6comprises an analog switch 10 which selectively interrupts the actuatingsignal C from the actuating signal generating means 7, a monostablemultivibrator 11 which responds to the output F from the comparator 9 soas to generate the output which lasts for a specified time interval, andan adder 12.

Next the mode of operation will be described. The command A from thecommand generating means 5 is transmitted through the analog adder 12 tothe actuating signal generating means 7 where the controlled variable Dfrom the controlled system 8 is subtracted from the secondary commandvariable B. The actuating signal C or the output of the actuating signalgenerating means 7 is applied through the analog switch 10 to the adder12 and directly to the controlled system 8. The comparator 9 alwayscompares the command A with the controlled variable D. When thesesignals coincide with each other, the comparator 9 applies its output tothe monostable multivibrator 11 so that the latter generates the outputwhich lasts for a specified time interval. In response to the outputfrom the monostable multivibrator 11, the analog switch 10 interruptsthe actuating signal C and generates the zero or constant output so asto reduce the difference between the command A and the secondary commandvariable B to zero. Alternatively, a constant is added to the command A.Thus the overshoot of the controlled variable D over the command A maybe prevented.

Next referring to FIG. 7, the analog circuit components of which areconstituted the servo system shown in FIG. 6 will be described indetail. The servo system shown in FIG. 7 has the function of bringingthe rotational speed of a motor to a set point. The command voltagerepresenting a desired rotational speed of the motor is set by sliding aslider 13' of a variable resistor 13 in the command generating means 5.The actuating signal from the actuating signal generating means 7 isapplied to one of the inputs of the analog switch 10. To the other inputis applied through a variable resistor 10' a 0 volt voltage representingthe value obtained by dividing or a command voltage by the gain of thecontrolled system 8. In response to the control input from themonostable multivibrator 11 which is of the retriggerable type, theanalog switch 10 gives the zero voltage or the constant voltage to theadder 12. The adder 12 comprises an operational amplifier 14 andresistors and has the function of adding to the command A the actuatingsignal C, the zero voltage or the constant voltage. The actuating signalgenerating means 7 comprises an operational amplifier 15 and itsassociated resistors and has the function of generating the actuatingsignal C which is the sum of the secondary command variable B and thecontrolled variable D which is inverted.

The controlled system 8 comprises an amplifier 16, a motor 17 therotational speed of which is controlled, a tachogenerator 18 adapted tomeasure the rotational speed of the motor 17 and an inverting amplifier20 comprising an operational amplifier 19 and its associated resistorsso as to invert the output from the tachogenerator 18, this invertedoutput being the controlled variable D.

The comparator 9 comprises an adder 22 which in turn comprises anoperational amplifier 21 and its associated resistors, ananalog-to-digital converter with sampling hold 23, PROM 24, a gate 25, amonostable multivibrator 26, a clock generator 27 and monostablemultivibrators 28 and 29. The adder 22 adds the command A to thecontrolled variable D which is inverted in polarity. The monostablemultivibrator 28 generates "L" pulse with a 6 microsecond pulse width inresponse to the negative going edge of the output of the clock generator27. In response to the positive edge of the output from the monostablemultivibrator 28, the monostable multivibrator 29 generates "H" pulsewith a 1 microsecond pulse width. In response to the 6 microsecond "L"pulse of the monostable multivibrator 28, the A/D converter 23 samplesand holds the result obtained by subtracting the controlled variable Dfrom the command A and starts the A/D converstion in response to theone-microsecond "H" control pulse from the monostable multivibrator 29so that the output may be derived from the A/D converter 23 after 8microseconds. Thus, the analog signal ranging from +10V to +0.02V isconverted into the digital signal represented by "000H" (H indicates thehexadecimal notation) to "1FEH"; the analog signal ranging between-0.02V and -10V, the digital signal between 200H and 2FEH; and theanalog signal 0V, the digital signal "1FFH".

PROM 24 (programmable read only memory) is so programmed that inresponse the address input "1FEH", "1FFH" or "200H", "H" output signalmay be derived. Next referring further to FIG. 8, the mode of operationof the first embodiment will be described. In the wave forms shown inFIG. 8, the polarities of the signals are not necessarily same as FIG. 7and are shown in such a way that their physical relations may be readilyunderstood. It is assumed that until T₁ there is no difference at allbetween the command A and the controlled variable D. When a step changein the command A occurs at T₁, the output or the secondary commandvariable B of the secondary command variable generating means 6 reachesa new secondary command variable A +0, which is equal to the newcommand. The output or the actuating signal of the actuating signalgenerating means 7 which is obtained by subtracting from the secondarycommand variable B the controlled variable D or the negative output fromthe inverting amplifier 20 of the controlled system 8, is applied to thecontrolled system 8 so that the controlled variable D approaches thecommand A. In response to the negative edge of the status signalrepresentative of the end of conversion by the A/D converter 24 afterT₁, the output of the A/D converter 24 is established, and it isdetected that there is a difference between the command A and thecontrolled variable D. As a result, the output from the gate 25 remains"L". Since no trigger pulse is derived from the gate 25, the monostablemultivibrator 11 in the secondary command variable generating means 6 isreset at T₂ so that the output changes to the "H" level. From T₂ theoutput from the analog switch 14 is switched to the actuating signal Cso that the output from the adder 12 in the secondary command variablegenerating means 6 becomes the sum of the command A and the actuatingsignal C. That is, the secondary command variable B increases.

At T₃ the difference between the command A and the inverted controlledvariable D becomes 0±1. After the output from the A/D converter has beenestablished, in response to the trigger pulse from the gate 25, themonostable multivibrator 11 in the secondary command variable generatingmeans 6 is triggered so that the output therefrom remains at the "L"level. In response to this "L" level output, the analog switch 10 addsthe input 0V to the adder 12 through the variable resistor 10'. As aresult, at T₄, the secondary command variable B approaches the command A+0 so that the overshoot of the controlled variable D may be avoided. Ifthe monostable multivibrator 11 is not retriggered its output isrestored to "H" level as indicated by the broken lines.

Also, the monostable multivibrator 11 holds the previous state at aperiod when the A/D converting outputs are yet indetermined. When thecontrolled variable D deviates from the command A, for example, bydisturbance the operation proceeding from the time T₂ is repeated fromT₅.

In the prior art servo system without the comparator 9, the monostablemultivibrator 11 and the analog switch 10, after T₄ the secondarycommand variable B is such that the controlled variable D is caused toincrease as indicated by the broken lines so that the overshoot of thecontrolled variable D results as indicated by the broken lines. However,when the comparator 9 and the analog switch 10 are incorporated, theovershoot may be avoided, the transient or settling time may beshortened.

So far it has been described that when there is no difference betweenthe command A and the controlled variable D, the analog switch 10 picksup the 0V input from the variable resistor 10' so that the secondarycommand variable B becomes equal to the command A +0. When the gain ofthe loop from the actuating signal C to the variable command D in thecontrolled system 8 is known, the output applied instead of 0V to theanalog switch 10 may be previously set to the value obtained by dividingthe command A by the loop gain. Thus when the controlled variable D is A±1, the secondary command variable B or the output from the secondarycommand variable generating means 6 becomes

    the command+(command/loop gain).

That is, the variations in amplitude of the secondary command variable Bmay be reduced so that the ripple of the controlled variable D may beminimized accordingly.

FIG. 9 shows the block diagram of a second embodiment of the presentinvention wherein the rotational speed of a motor is subjected to thesampling control. The command signal generating means 5 comprises aresistor, and in response to the pulses applied to the terminal 5', thecommand A is set.

The secondary command variable generating means 6 comprises an adder 30and a register 31 to which are applied pulses through the terminal 31'in response to the variations in the command A. The actuating signalgenerating means 7 comprises a subtracter. The controlled system 8comprises a holding circuit, a digital-to-analog (D/A) converter, anamplifier, a servomotor, a tachogenerator and an analog-to-digital (A/D)converter. The comparator 9 comprises a comparison circuit 32 and a gate33.

The servo system further includes a switch 34 for selectivelyinterrupting the transmission of the secondary command variable B to theactuating signal generating means 7, a switch for selectivelyinterrupting the transmission of the actuating signal C to thecontrolled system 8, and a switch 36 for selectively switching thetransmission of the controlled variable D from the controlled system 8to the comparator 9 or to the actuating signal generating means 7. Theseswitches are so operatively interconnected that first the contact 36a ofthe switch 36 is closed, then the contact 36b of the switch 36 and theswitch 34 are closed and finally the switch 35 is closed. And the abovesequence of switching operations is cycled.

For the sake of simplicity of the explanation, it is assumed that thedigital quantity "1" of the controlled variable D represents 25 rpm, thegain of the controlled system 8 be 18, and the time constant be 0.4 sec.The digital quantities in the control loops are represented by thedecimal numbers.

The mode of operation of the second embodiment with the aboveconstruction and circuit constants is as follows. When the commandrepresents 2,000 rpm (25×80), its digital representation is "80". Thatis, in the stationary state, the command A is "80" and the controlledvariable is also "80". The register 31 in the secondary command variablegenerating means 6 stores

    (80/18)+80=85.

It is assumed that the command A be changed to "64" representing 1,600rpm. Then in response to the pulse signals applied to the terminals 5'and 31', the contents in the registers in both the command generatingmeans 5 and the secondary command variable generating means 6 arechanged to "64". When the contact 36a of the switch 36 is closed, thecontrolled variable D which is "80" is compared with the command A whichis now "64" in the comparator 9 or more specifically in the comparisoncircuit 32. Since there is a difference between the command "64" and thecontrolled variable "80", the gate 33 provides no signal in response towhich the contents in the register 31 in the secondary command variablegenerating means 6 may be changed. When the contact 36b of the switch 36is closed while the switch 34 is closed, the actuating signal generatingmeans 7 subtracts from the secondary command variable B which is now "64" the controlled variable D which is "80" so that the output or theactuating signal C becomes "-16". When the switch 35 is closed, theactuating signal C which is now "-16" is applied to the controlledsystem 8 so that the deceleration of the motor results. At the sametime, the adder 30 in the secondary command variable generating means 6adds the actuating signal C which is "-16" to the command A which is now"64" and the sum "48" is stored in the register 31 in the secondarycommand variable generating means 6. In the next sampling period whichfollows 20 microseconds after the sum has been stored in the register31, the contact 36a of the switch 36 is opened. Assume that at this timethe controlled variable D is reduced to "78". Then since there is adifference between the command A and the controlled variable D, the gate33 in the comparator 9 will not generate the signal in response to whichthe contents in the register 31 may be changed. When both the contact36a and the switch 34 are closed, the actuating generating means 7subtracts the controlled variable D which is now "78" from the secondarycommand variable B which is now "48" at the end of the precedingsampling cycle. The difference "-30" or the actuating signal C isgreater than the actuating signal C which was "-16" in the precedingsampling cycle so that the deceleration of the motor results again asthe switch 35 is closed. At the same time, the adder 30 in the secondarycommand variable generating means 6 adds the actuating signal C which isnow "-30" to the command A which is "64" and the sum is stored in theregister 31 in the secondary command variable generating means 6. Theabove described operation is cycled until the controlled variable Dbecomes "64"; that is, there is no difference between the command A andthe controlled variable D. Then the switch 36 closes its contact 36a sothat the comparison circuit 32 in the comparator 9 compares the commandA with the controlled variable D. The difference is now "0" so that thegate 33 generates the signal in response to which the contents in theregister 31 in the secondary command variable generating means 6 changesto "64" and the transfer of the output from the adder 30 to the register31 is inhibited.

When the contact 36b of the switch 36 and the switch 34 are closed, thecontents "64" in the register 31 in the secondary command variablegenerating means 6 and the controlled variable D which is now "64" aretransferred into the actuating signal generating means 7. Theirdifference is now "0" so that the actuating signal C which is now "0" isapplied to the controlled system 8 when the switch 35 is closed. Sincethe actuating signal C is "0", no driving torque is applied to the motorin the controlled system 8 so that the motor is gradually decelerated.

When the rotational speed of the motor deviates from its set point sothat the controlled variable D deviates from "64", the above describedoperation is cycled until the controlled variable D may approach thecommand A. When disturbance is applied to the motor due to thevariations in the load so that the rotational speed drops, the abovedescribed operation is cycled until the difference between the command Aand the controlled variable D becomes zero except for the operation ofsetting the command A in the register 31 in response to the pulsesignals applied to the terminal 31' or the operation of changing thecommand A.

FIG. 10 shows the control sequence when the command A is decreased. Inresponse to a step change in the command A, the secondary commandvariable B undershoots and the controlled variable D follows thesecondary command variable B. Since the secondary command variable B isforced to set to the same value as the command A the overshoot of thecontrolled variable D may be prevented when it approaches the command A.Unless the secondary command variable B is not forced to set to thevalue same as the command A, the overshoot of the controlled variable Dwould result as indicated by the broken line curve D' as the controlledvariable D follows the secondary command variable B. In this case, thesecondary command variable B would change as indicated by the brokenline curve B'.

FIG. 11 shows the control or the restoration to the specified rotationalspeed when disturbance applied to the system changes due to thevariations in the load on the motor. The secondary command variable Band the controlled variable D change as indicated by the solid linecurves B and D, respectively. Unless there is not provided means forforcibly setting the secondary command variable B to the same value asthe command A, the secondary command variable B and the controlledvariable D would change as indicated by the broken line curves B' andD', respectively.

Next referring to FIG. 12, the second embodiment of the presentinvention will be described in more detail. The command generating means5 comprises a switch 37 adapted to represent the command or a specifiedroational speed of a motor in 8-bits and a D flip-flop 38 adapted tohold the command A in response to the timing signal. In response to thecommand or the rotational speed expressed in 8-bits, the switch 37generates the output represented in the hexadecimal notation system asfollows:

    ______________________________________                                        rpm                output from switch 37                                      ______________________________________                                        3 150      3 175       FFH                                                    1 975      2 000       DOH                                                    1 575      1 600       COH                                                    0          2.5         80H                                                    -2.5       0           7FH                                                    -3.2       -3.175      00H                                                    ______________________________________                                    

The double-track channels shown in FIG. 12 transmit 8-bits controlsignals.

The secondary command variable generating means 6 comprises a Dflip-flop 39 for holding the command A, a code converter 41 fordepressing an overflow of the added value, a comparator 42 whichcompares the output from the flip-flop 38 with that from the flip-flop39 and generates the output "1" when they coincide with each other orthe output "0" when they does not coincide or when the command A hasbeen changed, a second D flip-flop and a gate, a edge trigger Dflip-flop 43 comprising a D flip-flop and a gate and adapted to betriggered by the positive edge of the output of the comparator 42, agate 44 which generates the output "1" when either the flip-flop 43 orthe comparator 9 gives the input "1" to it, an 8-bits switch 45 adaptedto generate the signal representative of the value "00H" which is to beadded to the command A or the value obtained by dividing the referenceinput by the loop gain of the controlled system 8 which is also to beadded to the command A, an 8-bits adder 46 for adding the output fromthe switch 45 to the command A or the output from the flip-flop 39, adata selector 47 which selects the output from the adder 46 as an outputwhen the output from the gate 44 is "1" or selects the output from theconverter 41 as an output when the gate 44 delivers the signal "0", anda D flip-flop 48 which holds the output from the selector 47 in responseto the timing signal, the contents in the flip-flop 48 being thesecondary command variable B which has been established.

The actuating signal generating means 7 comprises a D flip-flop 49adapted to hold the controlled variable D in response to the timingsignal, a subtracter 50 for subtracting the output of the flip-flop 49from the output of the flip-flop 48 in the preceding stage 6, a codeconverter 51 adapted to depress an overflow of the subtracted value, a Dflip-flop 52 adapted to hold the output from the converter 51 inresponse to the timing signal, a switch 53 adapted to generate thesignal representative of "80H" and an adder 54 adapted to add the outputfrom the flip-flop 52 to the output from the switch 53.

The controlled system 8 comprises a digital-to-analog converter 55 forconverting the analog output from the adder 54 into a digital quantity,a power amplifier 56 for driving a motor 57, the motor 57 which is to becontrolled, a tachogenerator 58, a sample holding circuit 59 which isadapted to pass the output from the tachogenerator 58 so as to smooth itand amplify the smoothed output through a buffer amplifier, therebyeffecting the sample holding in response to the timing signal, and ananalog-to-digital converter 60 adapted to converting the analog outputfrom the sample holding circuit 59 into a digital quantity. The outputfrom the A/D converter 60 comprises 8-bits. The above describedrelations between the rotational speeds in rpm and their hexadecimalnotations described above may be also held in this system.

The servo system further includes a timing signal generator 61 forgenerating the control signals which control the command generatingmeans 5, the secondary command variable generating means 6, theactuating signal generating means 7 and the controlled system 8. Itcomprises a counter 62, an input data generator 63 for generating theinput data to be stored in the counter 62, a clock pulse generator 64, agate 65 for controlling the transmission of clock pulses from the clockpulse generator 64 to the counter 62, a decoder 66 for generating timingsignals by receiving the 4 bits output control signal from the counter62, a switch 67 for starting the control, a control signal generator 68adapted to generate not only the external load signal ("0" pulse) to beapplied to the counter 62 but also the control signal for controllingthe gate 65, a power on-off switch 69 and a clear signal generator 70adapted to generate the signals in response to which the flip-flops andthe counters are reset after the on-off switch 69 has been thrown, theclear signals being applied to the clear terminals of the flip-flops andcounters. It is to be noted that in order to stop the motor 57, thecommand A is changed to "80H" which represents 0 rpm.

The comparator 9 comprises a D flip-flop 71 adapted to hold thecontrolled variable D transmitted from the A/D converter 60 and acomparison circuit 72 adapted to compare the controlled variable D withthe command A so that when they coincide with each other it generatesthe output "1" at its output terminal.

Next the mode of operation of the servo system with the above describedconstruction will be described. The command A is held in the flip-flop38 in the command generating means 5 and also in the flip-flop 39 in thesecondary command variable generating means 6. The comparator 42compares the contents stored in the flip-flops 38 and 39 so as to detectwhether or not they coincide with each other; that is, whether or notthe command A has been changed. When the command A remains unchanged,the adder 40 adds the command A to the actuating signal C which is theoutput from the D flip-flop 52 in the actuating signal generating means7 at the end of the preceding sampling cycle. Any overflow of the addedvalue is detected from the most significant bit (MSB) of the output ofthe command A and the actuating signal C and the carry signal output ofthe adder 40 in the code converter 41, and the added value iscompensated to depress the overflow. The comparator 9 compares thecommand A with the controlled variable D from the A/D converter 60. Whenthey are coincident with each other, the selector 47 selects the outputfrom the adder 46 which adds the output "00H" from the switch 45 to thecommand A. When they are not, the selector 47 selects the output fromthe converter 41 which is the sum of the command A and the actuatingsignal C. The selected output or the actuating signal B is stored in theD flip-flop 48 and is subtracted from the controlled variable stored inthe flip-flop 49 in the subtracter 50 in the actuating signal generatingmeans 7. Any overflow of the added value is detected from the mostsignificant bit (MSB) and the carry output of the subtracter 50 and theadded value is compensated to depress the deviation and the depressedand added value is stored in the D flip-flop 52 of the actuating signalgenerating means 7. The output from the converter 51 is stored in theflip-flop 52.

In the subtraction carried by the subtracter 50, when the output fromthe flip-flop 48 or the secondary command variable B is smaller than theactuating signal C which is a subtrahend, the output from the subtractor50 is a 2's complement of the difference. The relations between thedifferences in the decimal notation system and their hexadecimalnotations are as follows:

    ______________________________________                                        difference in decimal                                                                         hexadecimal notations of                                      notation system the output from the subtracter 50                             ______________________________________                                        127             7FH                                                           1               01H                                                           0               00H                                                           -1              FFH                                                           -2              FEH                                                           -128            80H                                                           ______________________________________                                    

Therefore when the sum of the decimal numbers becomes negative, theoutput from the adder 40, which adds the command A to the actuatingsignal C, becomes (the command A) minus (the absolute value of theactuating signal C).

The adder 54 adds the output "80H" from the switch 53 to the output fromthe flip-flop 52. The relation between the output of the adder 54 andrevolution number makes same as that between the command A of the switch37 and revolution number, and when the output of the D/A converter ofthe controlled system 8 is 0 or the actuating signal C is 00H, thecontrolled system 8 is controlled at a relation in which the motor doesnot produce any torque.

So far it has been described that when the controlled variable Dapproaches the command A, switch 45 delivers the output "00H" to theadder 46 so as to be added to the command A, thereby obtaining thesecondary command variable B. However, when the gain of the loop fromthe actuating signal C to the controlled variable D is known, the switch45 may be so arranged as to deliver the output representative of

    (command A)/loop gain

whereby when the command A has been changed or when the controlledvariable D has approached the command A, the secondary command variableB becomes

    (command A)+(command a/loop gain)

Then, the variations in amplitude as shown in FIG. 10 may be minimized,whereby the ripple of the controlled variable D may be minimizedaccordingly.

In this embodiment, first whether or not the controlled variable D isequal to the command A is detected and when the controlled variable Dapproaches the command A, the secondary command variable B is set to thecommand A. When the controlled variable D deviates from the command A,the actuating signal C is added to the command A. Thus the controloperation is carried out.

In a third embodiment of the present invention, when the controlledvariable D approaches the command A after the latter has been changed,the secondary command variable B is changed to the command A andthereafter will remains unchanged as will be described in detail below.

Referring to FIG. 13, the command generating means 5, the secondarycommand variable generating means 6, the actuating signal generatingmeans 7, the controlled system 8 and the comparator 9 are similar inboth construction and mode of operation to those shown in FIG. 9 so thatno further description thereof shall be made. When the command A ischanged, a new command A is set into the register 31 by applying thepulses at the terminal 31'. The command change signal is applied to theterminal 5' of the command generating means 5 so that "01H" fed from theterminal 73' may be set in a counter 73. When "01H" is set in thecounter 73 and the comparison circuit 32 of the comparator 9 detects thecoincidence between the command A and the controlled variable D, thecoincidence signal of the comparison circuit 32 is transmitted throughthe gate 33 to the secondary command variable generating means 6 and thecounter 73, the command A is set in the secondary command variablegenerating means 6, the content of the counter 73 is subtracted unityand set 00H and the gate 33 is closed. Therefore, it is detected whetheror not the controlled variable D has approached the command A after thelatter has been changed and then the servo system becomes the operationsame as the servo system in the prior art as shown in FIG. 1. Then byone operation described above, when the level of disturbance applied tothe controlled system 8 is low, the ripple may be avoided and the steadystate error may be reduced to a minimum.

Referring further to FIG. 14, the third embodiment will be described inmore detail. FIG. 14 shows part of the embodiment shown in FIG. 13 andthe same reference numerals are used. The counter 74 shown in FIG. 14comprises a D flip-flop 75 adapted to deliver the output "1" when theoutput pulse from the adder 42 is "0" when the command A is to bechanged, a gate 76 which delivers the output "0" when the inputs fromthe flip-flop 75 and the comparison circuit 72 and the timing signal areall "1", a count-down counter 77 which sets "01H" in response to the "0"output pulse from the gate 74, a switch 78 adapted to generate thesignal representative of the modifying number or unity a decoder 79comprising a decoder section and a gate which output is unity whenmodifying numbers and 0 are set in its terminals, a gate 80 forgenerating output pulse rising in the back edge of the timing pulse whenthe output of the D flip-flop 75 is unity and the output of thecomparison circuit 72 of the comparator 9, and a gate 81 which deliversthe output pulse "0" when the output from the comparison circuit 72 is"1", the output from the decorder 79 is "0" and the timing signalexsists and resets the D flip-flop 73 after the modifying numbers aresatisfactory.

While in the servo system shown in FIG. 13, the secondary commandvariable B is approached only once to the command A after the latter hasbeen changed, in the third embodiment shown in FIG. 14, the switch 78permits the secondary command variable B to approach the command A asmany times as required.

What is claimed is:
 1. A servo system comprisinga command generatingmeans, a secondary command variable generating means for generating asecondary command variable by adding an actuating signal to the command,an actuating signal generating means for generating said actuatingsignal by subtracting from said secondary command variable a controlledvariable which represents the output of a controlled system, saidactuating signal being applied to said controlled system, a comparatormeans adapted to compare said command with said controlled variable,whereby when said controlled variable approaches said command, acoincidence signal is delivered from said comparator means to saidsecondary command variable generating means so as to change the outputof said secondary command variable generating means.
 2. A servo systemas set forth in claim 1 wherein said secondary command variable meansincludes switching means connected to setting signal generating meansfor generating desired setting signals in response to the coincidencesignals from said comparator means, outputs of said setting signalgenerating means being added to the command to become said secondarycommand variable.
 3. A servo system as set forth in claim 2 wherein saidsetting signal generating means includes means for generating zerosignals.
 4. A servo system as set forth in claim 2 wherein said settingsignal generating means includes means for generating the resultobtained by dividing the command by the loop gain from the output of theactuating signal generating means through the controlled system to theinput of said actuating signal generating means.
 5. A servo system asset forth in claim 1 wherein said secondary command variable generatingmeans includes a monostable multivibrator which generates a signal whichlast for a specified time interval when said secondary command variablegenerating means receives the coincidence signal from the comparatormeans and a switch for selecting values addible to said command during asetting time in response to the output from said monostablemultivibrator.
 6. A servo system as set forth in claim 1 furthercomprising a first switch adapted to selectively switch the applicationof said controlled variable to said comparator means or said actuatingsignal generating means, a second switlch interposed between saidsecondary command variable generating means and said actuating signalgenerating means, and a third switch interposed between said actuatingsignal generating means, the controlled system and said secondarycommand variable generating means, whereby said first switch transmitssaid controlled variable to said comparator means which compares saidcommand with the controlled variable, thereafter the first switch is soactuated as to transmit said controlled variable to said actuatingsignal generating means, thereafter said second switch is closed so asto cause said actuating signal generating means to subtract saidcontrolled variable from said secondary command variable and finallysaid third switch is closed so as to transmit said actuating signal fromsaid actuating signal generating means to said secondary commandvariable generating means and to said controlled system.
 7. A servosystem as set forth in claim 1 further comprising a counter which is setto "1" in response to the command change signal applied to said commandgenerating means and a gate circuit which is responsive to the outputfrom said counter for transmitting the output from said comparatormeans, whereby in response to said coincidence signal from saidcomparator means the output from said secondary command variablegenerating means is caused to coincide with said command and at the sametime the output of said counter is set to "0", thereby interrupting theoutput from said gate circuit and changing only once the output fromsaid secondary command variable generating means to said command afterthe latter has been changed.